The quality of printing on single sheets is often unsatisfactory. One reason, among others, for this resides in the sheetfeeding apparatus. If the sheet doesn't lie in fully extended position on the feeder, i.e. on the front gauges, it is grasped in a wavy or warped condition and fed that way to the first printing unit. Between the printing cylinders, the sheet is pulled or gathered, stretched, and perhaps also accordioned lengthwise. This can lead to ghosting. If, for example, a first sheet lies flat on the feeder, while the next sheet lies warped or wrinkled, this leads inevitably to differing sheet positions in the particular printing units. If one sheet to be printed has passed through the first printing unit and is fed to the second printing unit, it has more or less undergone a relaxation process, i.e. the first color is transferred in the second printing unit to a different position, which causes a loss of print quality.
There are a number of devices generally known which seek to achieve a flat orientation of the sheets and their leading edges upon feeding into the sheetfeeder, and to avoid or eliminate waviness or warping. German Pat. No. 27 13 994, Fischer, assigned to a related company of the assignee of this application, discloses a paper stretching apparatus which pulls the leading corners, of a sheet grasped by the grippers, away from each other sidewise. Such paper stretching devices are, indeed, relatively effective in correcting the aforementioned problem, but, due to the extensive mechanical effort required, they are relatively expensive.